The present invention relates to a superconducting device, and more particularly to a superconducting device suitable for use in a nuclear magnetic resonance computer tomography apparatus (hereinafter referred to as "NMR-CT apparatus") for medical treatment.
It is well known that when metals such as NbTi and Nb.sub.3 Sn are cooled to a very low temperature of about 4.2.degree. K., the resistance of the metals becomes equal to zero, that is, these metals are put in a superconducting state. This phenomenon is utilized for generating a stable, strong magnetostatic field without any power loss.
A superconducting magnet which is formed by using one of the above metals and can generate a strong magnetostatic field stably without producing any power loss, is widely used, as a magnetostatic field generating device, in various industrial apparatuses including an NMR-CT apparatus, a magnetic floating train, and a charged particle focussing apparatus. Specifically, a superconducting coil is most suitable in an NMR-CT apparatus which requires a highly-uniform, highly-stable magnetostatic field, and hence attention has been paid to the superconducting coil in recent years.
In the above industrial apparatuses, a horizontal type superconducting device is widely used in which the central axis of magnetic field generated by a superconducting coil is made parallel to a horizontal direction in order to make it easy to utilize a space where a magnetostatic field is established by the superconducting coil. In more detail, a superconducting magnet can operate stably only in liquid helium (of 4.2.degree. K.) acting as a coolant, and hence is placed in a helium vessel containing liquid helium so as to be immerred in the liquid helium. In this case, it is necessary to prevent heat from the surroundings of ordinary temperature (namely, about 300.degree. K.) from entering into the liquid helium. Usually, a gas helium shield plate of about 20.degree. K. is provided around the helium vessel, a liquid nitrogen shield plate of about 80.degree. K. is provided around the gas helium shield plate, and the helium vessel and the shield plates are placed in a heat-insulating vacuum vessel. Thus, the rate of evaporation of liquid helium which is expensive, is made as small as possible. (Means for maintaining a very low temperature state is disclosed in, for example, Japanese patent examined Publication No. 54 -43359 published on Dec. 19, 1979.)
While, in order to operate a superconducting coil, it is necessary to cool the superconducting coil to the liquid helium temperature and to cause an exciting current to flow through the superconducting coil thus cooled. Further, since the amount of liquid helium contained in a helium vessel decreases due to evaporation, it is necessary to resupply liquid helium and to discharge evaporated helium. Accordingly, a superconducting device is usually provided with a pipe for introducing liquid helium into a helium vessel, a pipe for discharging evaporated helium, and a power lead for supplying a current to a superconducting coil. The liquid helium introducing pipe, the helium gas discharge pipe, and the power lead are led to the outside through a liquid helium inlet port provided on the superconducting device.
The conventional, horizontal-type superconducting device is provided with the liquid helium inlet port, at the top part thereof viewed in vertical directions. This is because, when liquid helium is poured from the top part in a vertical direction as in an ordinary liquid storage device, a helium vessel can be filled with liquid helium to the top thereof. That is, in the conventional, horizontal-type superconducting device, liquid helium is introduced into the helium vessel through the liquid helium inlet port provided at the top part of the superconducting device. In order to supply liquid helium to the helium vessel from the liquid helium inlet port provided at the top part, it is required for an operator to couple a transfer tube from a liquid helium tank (namely, a Dewar vessel for storing liquid helium) with the liquid helium inlet port through the aid of a ladder or the like, thereby pouring liquid helium stored in the liquid helium tank, from above the inlet port into the superconducting device.
For such a superconducting device, however, it is required that a space for inserting the transfer tube into the liquid helium inlet port exists above the inlet port, since the inlet port is provided at the top part of the superconducting device. Usually, that portion of the transfer tube which is inserted into the liquid helium inlet port, couples the inlet port of ordinary temperature with the vacuum vessel of 4.2.degree. K. In order to make as small as possible the amount of heat conducted from the outside to the helium vessel, it is required to make the inserted portion of the transfer tube sufficiently long. Thus, in order to supply liquid helium from the liquid helium tank to the helium vessel, it is required that a work space for inserting the transfer tube into the liquid helium inlet port exists above the inlet port and moreover has roughly the same height as the inlet port. The necessity of such a work space arouses a serious problem when the superconducting device is large in size. In more detail, the NMR-CT apparatus is usually used in a hospital or the like, and the problem arises when a large-sized superconducting device included in the NMR-CT apparatus is installed in a small room of the hospital. In other words, when the large-sized superconducting device is placed in the small room, it is difficult to secure a work space for supplying and resupplying liquid helium to a helium vessel.